Optical fiber based sensing systems are known already in the art. OTDRs are used to determine fiber condition and properties, such as splice or connector losses and attenuation, whereas DAS and DTS systems use backscatter and/or reflections from along the fiber to sense acoustic energy incident on the fiber, or ambient temperature around the fiber, as appropriate. An example prior art DAS system is the Silixa® iDAS™ system, available from Silixa Ltd, of Elstree, UK, the details of operation of which are described in our earlier patent application WO2010/0136809, any details of which that are necessary for understanding the present invention being incorporated herein by reference. An example DTS system is the Silixa® Ultima™ system.
At a high level, DAS and DTS systems operate by sending sensing pulses down an optical fiber deployed in the environment which is to be monitored. For a DAS system the vibrations of an incident acoustic wave on the fiber cause modulations in the backscatter or reflections from the fiber as the pulse travels along the fiber. By measuring the backscatter and/or reflections and detecting such modulation then the incident acoustic wave can be determined. For a DTS system, ambient temperature affects the amount of backscatter and/or reflections from different parts of the fiber at different ambient temperatures, so that again temperature along the fiber can be inferred by monitoring the backscatter and/or reflections.
At present most optical fiber DAS and DTS systems are limited in range to around 5 km or so, due to attenuation in the fiber of both of the outward sensing pulse, and the resulting backscatter and/or reflections along the fiber. Specifically, as a sensing pulse travels along the fiber it will spread in time, and decrease in amplitude (and power), such that backscatter and/or reflections from along the fiber from the pulse will consequentially also be temporally spread and be of lower amplitude. Given that the backscatter and/or reflections will need to travel back along the fiber to the DAS sensor, and hence be further dispersed by the fiber, there is a limit to the range of fiber along which a pulse can be sent and resulting backscatter and/or reflections determined before the backscatter and/or reflections hit the sensor noise floor. In a typical DAS or DTS scenario, a range of around 5 km would be typical i.e. the DAS or DTS would be able to resolve a signal along approximately 5 km of sensing fiber.
For many DAS or DTS sensing applications, a 5 km range is more than adequate. However, for some applications, and particularly security applications such as pipeline security or area security, a greater range would be useful. Whilst range can of course be increased by the provision of several independent systems (i.e. it would be possible to position a DAS box every 5 km along a pipeline), such increases the system deployment cost, and leads to other problems in synchronisation of monitoring of several independent sensor systems of the same type. It would therefore be highly beneficial if the range of optical fiber sensing systems, such as DAS and DTS, but including other OTDR systems, could be increased.